A general electrophotographic image-forming method provides a toner image as described below by utilizing, for example, a photoconductive substance. An electrical latent image is formed on an electrostatic latent image-bearing member by various means. Next, the latent image is visualized by being turned into a toner image through development by a developing apparatus. Next, the toner image is transferred onto a transfer material such as paper as required, and is then fixed with heat, pressure, heat and pressure, or solvent vapor. An image-forming apparatus for such method is, for example, a copying machine or a printer.
A reduction in the size of the main body of a copying machine or printer employing the electrophotographic method has been requested in recent years in consideration of energy savings and space savings. In addition, meanwhile, such high durability as described below has been requested of the copying machine or printer. Namely, it is required that no reduction in image quality occurs even after images have been copied or printed on a large number of sheets.
One method for the reduction in the size of the main body of any such image-forming apparatus is the simplification of a fixing apparatus. The simplification of the fixing apparatus is, for example, film fixation that facilitates the simplification of a heat source and the construction of the apparatus. In the film fixation, the simplification of the heat source and the construction of the apparatus are facilitated. In addition, good thermal conductivity is obtained as a result of the use of a film as a fixing member. Accordingly, a first printout time can be shortened. However, the film is used while being pressed against a roller under a relatively high pressure, and hence a problem such as the wear of the film at the time of its long-term use is apt to arise.
A toner that shows good fixability even under a light pressure has been requested for suppressing such problem. In addition, meanwhile, an ability to perform development with improved stability has been requested of the toner, and an improvement in terms of such developing performance as described below has also been requested of the toner. A high image density and high image quality can be obtained even at the time of its long-term use.
Investigations from various aspects such as a toner structure and the improvement of a release agent have been conducted on such problems as described above concerning, for example, the fixability of the toner and development stability at the time of its long-term use.
Proposed in Patent Literature 1 are a polymerized toner of such a core-shell type structure that core particles formed of colored polymer particles each containing a polyfunctional ester compound, a Fischer-Tropsch wax, and a coloring agent are each covered with a shell formed of a polymer having a glass transition temperature higher than the glass transition temperature of a polymer component that forms each of the core particles, in which the usage ratio between the polyfunctional ester compound and the Fischer-Tropsch wax is 5/5 to 29/1, and a method of producing the toner.
In addition, Patent Literature 2 proposes a method of producing a toner including polymerizing a polymerizable monomer composition having at least a polymerizable monomer and a coloring agent in an aqueous medium, the method of producing a toner being characterized in that a peroxide-based initiator of a dicarbonate type is used as a polymerization initiator.
In addition, Patent Literature 3 proposes a magnetic toner having toner particles each containing at least a binder resin, a wax, and a magnetic powder, and an inorganic fine powder, the magnetic toner being characterized in that the toner particles have an average circularity of 0.960 or more, that substantially no magnetic powder is exposed to the surface of each toner particle, and that the wax has at least two endothermic peaks in differential calorimetry, one of the endothermic peaks is present in the range of 40 to 90° C., and the other is present in the range of 70 to 150° C.
Although fixability is improved in an ordinary fixing unit construction by each of those toners, each of those toners has showed insufficient fixability in the film fixation of a light-pressure type like the present invention. In addition, the following new problem has become recognized. The releasability of each of the toners from the fixing member reduces probably owing to the fact that the fixing unit construction of the present invention is of a light-pressure type, and hence the contamination of the fixing film occurs. Further, the toners each still have had room for improvements in image density and image quality at the time of its long-term use as well.